1. The Field of the Invention
The invention relates to an orthogonal multicarrier transmission system and corresponding method applying a pulse shaping filter to mitigate intercarrier and intersymbol interference. In particular the invention relates to an orthogonal multicarrier transmission system using an Offset-QAM modulation and applying a non-symmetric conjugate-root (CR) filter for pulse shaping, which will be referred to by the term Conjugate-Root OQAM (CR-OQAM).
2. The Relevant Technology
Current 4G LTE-(Advanced) systems are based on OFDM that provides intersymbol interference (ISI)-free and intercarrier interference (ICI)-free transmissions in ideal AWGN channels, i.e. in channels where only white Gaussian noise is added to a transmit signal. However, if the channel is time-frequency dispersive such as a real channel of a cellular communication system, good time-frequency localization of the transmit signal is required to cope with asynchronicities. Furthermore, low out-of-band radiation is required to enhance spectral agility and aggregation of carriers.
One way to mitigate interference in transmissions is to use so-called Offset-QAM (OQAM) instead of conventional QAM modulation in a multicarrier system, for example OFDM/OQAM. In Offset-QAM modulation complex valued data symbols ck,m are transmitted on 1 . . . k . . . K−1 subcarriers, where the real and imaginary portion of a symbol are offset by half a symbol duration T, i.e. by ½T. To mitigate ISI and ICI, each symbol is pulse-shaped with a symmetric, real-valued half-Nyquist pulse shaping filter which is band-limited to two subcarriers.
While this approach does not achieve complex orthogonality, it provides quasi-orthogonality in the real domain. With regard to the symbol rate, these systems allow doubling of the symbol rate, but only real-valued symbols are transmitted. Hence these systems provide the same effective data rate as conventional OFDM/QAM systems. However, well localized transmit and receive filters can still be used to cope with misalignments in time and frequency.
The application of conventional Offset-QAM in multicarrier systems requires the use of pulse shaping filters, wherein these have to be symmetric in both the time and the frequency domain and furthermore have to be half-Nyquist filters. Furthermore, in conventional Offset-QAM systems, a phase shift of ½π is required between subcarriers and subsequent symbols, i.e. adjacent sub-carriers are shifted in phase by ½π against each other. As a consequence the phase space of conventional multicarrier systems using Offset-QAM is varying in both time and frequency and thus increases the complexity of an implementation in a transmitter and corresponding receiver.
Hence there is a need to provide an improved Offset-QAM system that at least mitigates some of the above mentioned problems.